LIGHTNING 367

the column of air through which it passes, thereby shattering chimneys, ripping off shingles, and producing many other similar, and surprising, results. It also explosively vaporizes such volatile objects as it may hit, that have not sufficient conductivity to carry it off. Hence, trees are stripped by it of their bark or utterly slivered and demolished through the sudden volatilization of sap and other substances; wire is fused and vaporized, and even the vapor blown through woven insulation without burning it; holes are melted through steeple bells and other large pieces of metal; and a thousand other seeming freaks and vagaries are wrought-

Many of the effects of lightning appear, at first, difficult to explain, but, except the physiological, which, indeed, are but little understood, and, probably, some of the chemical, nearly all depend upon the sudden and intense heating along its path.

Crushing Effects.—One of the more surprising phenomena of the lightning discharge is the crushing of hollow conductors, an effect that gives some idea of the strength of current and quantity of electricity involved, and, therefore, deserves a full discussion.

Pollock and Barraclough1 have described and explained this phenomenon in connection with a hollow copper cylinder; outside diameter 18 nun., inside 16 mm., lap joint 4 mm. wide, 2 mm. thick. In what follows, however, reference will be had to a remarkable, and even more instructive, product of the same phenomenon, kindly lent by West Dodd, of Dos Moincs, Iowa. Figure 136 shows two originally duplicate (so reported), hollow, copper lightning rods, one uninjured (never in use), the other crushed by a discharge. The uninjured rod consists of two parts, shown assembled in Fig. 136, and separate in Fig. 137. The conical cap, nickel plated to avoid corrosion, telescopes snugly over the top of the cylindrical section, and when in place, where it is left loose or unsoldered, becomes the ordinary discharge point.

The dimensions are:

Soc.tkm Outside Diameter Inside Diaraete^

Cylinder..................... 16.0 mm. 14.65mm.

Con<> shank................... 17.4mm. 16.0 mm.

Length of conical cap, cylindrical portion, 7 cm., total 19 cm.

Both the cylindrical and the conical portions of the rod are securely brazed along square joints.

The general effects of the discharge, most of which are obvious from the illustrations, were:

1. One or two centimeters of the point were melted off.

2. The conical portion of the top piece and all the cylindrical rod except the upper 2 centimeters, roughly, within the cap, were opened along the brazed joint.

' J. and 1'roc. Roy. Soc., N. S. Wales, 39; 131, 1905.